This invention relates to the field of radio paging and more particularly to a system for encoding a page address into tone signals for transmission over telephone lines to a decoder and transmitter.
Prior art paging systems have encoded page addresses into DTMF (dual tone, multi-frequency) signals to be sent over telephone company lines, but these systems have required many unnecessary steps on the part of the operator, causing fatigue from listening to a variety of signals and responding properly to them. Errors are also possible under such circumstances, and time has to be allowed for operator reaction. Other systems have required a separate DC pair of telephone lines when using a dedicated line in order for the decoder terminal to sense an incoming call. This is an extra expense for the user as well as a disadvantage to the telephone company, since DC signaling is not possible over paths such as microwave lengths and certain switching points. Prior systems have had station identification transmitted automatically at regularly timed intervals, whereas an ID after each transmission is preferred by the FCC. Use of a telephone company-supplied tone encoder (a type of telephone) for providing DTMF signals has numerous disadvantages besides cost, e.g., a handset must be taken off-hook, held to monitor incoming signals and send voice messages, then placed on-hook after each transmission. The handset microphone, being "live" while off-hook can pick up noise which would interfere with transmission of DTMF, so that operator discipline is required. Other systems have required the use of an encoder for testing the decoder since the input of the decoder did not appear as a telephone to the telephone lines. Another problem in previous systems has been the varying line loss in various telephone paths.